Multiplex coherent raman spectroscopy detector and method

ABSTRACT

A multiplex coherent Raman spectrometer ( 10 ) and spectroscopy method rapidly detects and identifies individual components of a chemical mixture separated by a separation technique, such as gas chromatography. The spectrometer ( 10 ) and method accurately identify a variety of compounds because they produce the entire gas phase vibrational Raman spectrum of the unknown gas. This is accomplished by tilting a Raman cell ( 20 ) to produce a high-intensity, backward-stimulated, coherent Raman beam of 683 nm, which drives a degenerate optical parametric oscillator ( 28 ) to produce a broadband beam of 1100-1700 nm covering a range of more than 3000 wavenumber. This broadband beam is combined with a narrowband beam of 532 nm having a bandwidth of 0.003 wavenumbers and focused into a heated windowless cell ( 38 ) that receives gases separated by a gas chromatograph ( 40 ). The Raman radiation scattered from these gases is filtered and sent to a monochromator ( 50 ) with multichannel detection.

CROSS-REFERENCE TO RELATED PROVISIONAL APPLICATION

[0001] This application claims the benefit of the filing date ofprovisional application No. 60/254,926, filed Dec. 13, 2000.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with Government support under NationalScience Foundation grant CHE-9702087, NASA Faculty Awards for Researchgrant NAG3-1974 and NASA grant NCC3-758, and Department of Energy grantDE-FG01-96EW13219. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to a multiplex coherent Ramanspectroscopic detector and method for generating and detecting coherentRaman radiation from a sample. More specifically, the present inventionrelates to a multiplex coherent Raman radiation detector and method forgenerating and detecting coherent Raman radiation scattered by thecomponents of an unknown sample and using the coherent Raman radiationto determine the identity of the sample's constituents.

[0005] The present invention also relates to an apparatus and method forilluminating a gaseous sample with broadband light with a continuousrange of more than 3000 wavenumbers and with a narrowband light having abandwidth of less than 1 wavenumber, and preferably about 0.003wavenumbers to produce the entire gas phase vibrational Raman spectrumof the sample, thereby permitting accurate identification of the sample.

[0006] In addition, the present invention relates to an apparatus andmethod for increasing the intensity of the backward-propagating,phase-conjugate, coherent Raman radiation produced by a Raman cell.Moreover, the present invention also relates to an apparatus and methodfor using this enhanced, backward-propagating, phase-conjugate, coherentRaman radiation to drive a device capable of producing broadband lightof more than 3000 wavenumbers.

[0007] 2. Description of Related Art

[0008] In many fields, such as scientific research, industrial research,and forensics, it is often necessary to identify the chemicalcomposition of an unknown sample. This task is often performed by firstisolating the different compounds in the sample, and then applying anidentification technique to each isolated compound. One standard methodfor isolating unknown compounds is called gas chromatography, where theunknown sample is transformed into a gas, if not already in the gaseousstate, and the various compounds in the gas are separated due to theirdiffering gaseous properties, such as polarity. Once the compounds areisolated, they may be identified. The simplest way to identify thecompounds is by noting the time it takes for each compound to passthrough the gas chromatograph, since different compounds take differentamounts of time to do so. But this method is limited to samples wheremuch is known about the components. A more powerful method foridentifying isolated compounds examines the intensity of differentwavelengths of light emitted, transmitted, reflected, or scattered bythe compound. This technique, called spectroscopy, works if eachcompound emits, transmits, reflects, or scatters light differently andif the spectroscopic instrument has sufficient spectral resolution todetect these differences. More specifically, different chemicalcompounds emit, transmit, reflect, or scatter different wavelengths oflight with differing intensities. A graph or picture of such data iscalled the spectrum of that compound. Different types of spectroscopyreproduce the spectrum of a compound over different wavelengths and/orunder different conditions. If the type of spectroscopy used provides aunique spectrum for each chemical compound, an unknown compound can beidentified by producing its spectrum (for example, by illuminating thecompound and measuring the light reflected, scattered, or emittedtherefrom) and comparing its spectrum with the spectra of knowncompounds. As a result, gas chromatographs, which isolate compounds froma sample, are often used with spectrometers, which identify thecompounds once they are isolated.

[0009] One popular type of spectroscopy detector used with gaschromatographs requires the gas isolated by the gas chromatograph to beembedded in or condensed onto a substrate before spectroscopicexamination. Such detectors provide advantages, such as low detectionlimits, but are complicated because they require the isolated gas to becondensed, trapped, or adsorbed onto a substrate. In addition, suchdetectors suffer from unwanted effects such as nearest-neighbor effects,sample decomposition, and a slow detection speed. As a result, detectorsthat operate “on the fly” with little or no sample modification areoften faster and freer from unwanted effects.

[0010] One type of frequently-used “on the fly” spectroscopy is infraredspectroscopy. But infrared spectroscopy is sometimes unable toaccurately determine the identity of an unknown sample because certaincharacteristics of some samples (i.e., those with spectra that arehighly state- (phase) dependent and those that produce strong rotationalside bands in the infrared light absorbed by the sample that cause aloss of spectral resolution) reduce its accuracy. Furthermore, certainmolecules, such as homonuclear diatomics, have no infrared spectrum, andoptical components designed to direct and process the infrared lightused in an infrared spectrometer are often inferior to the opticalcomponents designed for use in the visible spectrum.

[0011] A type of spectroscopy that is less susceptible to these problemsis called Raman spectroscopy. In this type of spectroscopy, light in thevisible wavelength region or the near-visible wavelength region isprojected onto a sample and a small fraction of this light is scatteredin all directions by the sample and is measured. The light is scatteredbecause the molecules of the sample inelastically scatter the light dueto the vibrational or rotational motions in the molecules of the sample.Such scattered light is of two types: light whose wavelength is notshifted, which is called Rayleigh scattering, and light whose wavelengthis shifted, which is called Raman scattering. The Raman scattered lightis much less intense than the Rayleigh scattered light. Since the Ramanscattered light is scattered and shifted in wavelength because of thevibration of molecules of the sample, a graph of the Raman scatteredlight from a sample is called the vibrational Raman spectrum of thesample and provides information about the internal vibrational motion ofthe molecules of the sample. Moreover, the entire vibrational Ramanspectrum of each compound (which is approximately 3000 wavenumbers wide)is unique to that compound. As a result, unknown compounds can beidentified by their vibrational Raman spectrum. But, the intensity ofthe Raman spectrum must be sufficiently strong to be detected bycurrently-developed detectors with a high signal-to-noise ratio, and theentire Raman vibrational spectrum, covering a range of at least 3000wavenumbers (indicating a large number of wavelengths of light aremeasured) must be produced. If only a partial Raman vibrational spectrumis produced, the identity of the compound may not be determined withhigh accuracy, since many compounds can share the same partial Ramanvibrational spectrum. When Raman spectroscopy is used to detect gases,such as those isolated by a gas chromatograph, it is called gas phaseRaman spectroscopy.

[0012] Gas phase Raman spectroscopy provides several advantages over gasphase infrared spectroscopy. First, Raman spectroscopy is lesssusceptible to phase transitions in the sample and to unwantedbroadening of scattered or absorbed light due to rotational sidebands,so species identification may be more accurate using Raman spectroscopy.Second, Raman spectroscopy can be used to identify more types ofmolecules than infrared spectroscopy, since certain molecules do notappear in infrared spectroscopy, while all molecules will appear inRaman spectroscopy. Third, several advanced techniques are availablewith Raman spectroscopy that improve its accuracy and generateadditional, valuable data not available in infrared spectroscopy,including resonance Raman spectroscopy, surface enhanced Ramanspectroscopy, and coherent Raman spectroscopy. Finally, the opticalcomponents commercially for use in the visible region are often superiorto those available for use in the infrared region. For example,extremely sensitive and rapid multichannel detectors are available inthe visible region but not in the infrared region.

[0013] But gas phase Raman spectroscopy suffers its own problems. Thedensity of molecules in the gaseous sample is so low that longcollection times (minutes or hours) are needed in order to generateRaman spectra. This problem precludes the use of conventional Ramanspectroscopy as an on-the-fly detector for gas chromatography, since ingas chromatography, different gases emerge from the gas chromatographevery few minutes, seconds, or less.

[0014] In order to overcome this problem, Roth and Kiefer, in“Surface-Enhanced Raman Spectroscopy as a Detection Method in GasChromatography,” Applied Spectroscopy vol. 48, 1994, 1193-1195, exploredthe potential use of surface enhanced Raman spectroscopy (SERS). Surfaceenhancement can be used to increase the strength of the Raman signal,thereby reducing the time required to obtain spectra. Later, Carron andKennedy published the first paper showing actual chromatograms using aSERS detector in “Molecular-Specific Chromatographic Detector UsingModified SERS Substrates,” Analytical Chemistry vol. 67, 1995,3353-3356. Their method requires that the sample be trapped onto asubstrate that attracts specific molecules based on their functiongroups and enhances them. This method offers high sensitivity andspecificity. But it also suffers important disadvantages including thedomination of the spectra by the substrate instead of the sample, thelack of universality of the technique (not all molecules will stronglyadsorb onto a given substrate, and not all molecules will be enhanced),the frequent replacement of the substrate, and the possibledecomposition of the sample or possible change of the sample uponadsorption onto the substrate.

[0015] To solve these problems with gas phase Raman spectroscopy,coherent Raman spectroscopy was developed in the early 1960s. Unlikeconventional Raman spectroscopy and SERS, coherent Raman spectroscopyuses two or more pulsed lasers having sufficiently high peak intensitiesto cause a certain nonlinear optical effect in the sample that generatesan intense, coherent beam in one direction. In contrast, in conventionalRaman spectroscopy and in surface enhanced Raman spectroscopy, thesignal is weakly scattered in all directions. This technique isdescribed in “Multiplex Coherent Anti-Stokes Raman Spectroscopy by useof a Nearly Degenerate Broadband Optical Parametric Oscillator”, AppliedOptics, vol. 38, no. 27, pp. 5894-5898, Sep. 20, 1999 by Peter C. Chen,Candace C. Joyner, and Michael Burns-Kaurin, and “Improved ScanningRange for Coherent Anti-Stokes Raman Spectroscopy Using A TunableOptical Parametric Oscillator”, Analytical Chemistry, col. 68, no. 17,pp.3068-3071, Sep. 1, 1996 by Peter Chen, both of which are incorporatedby reference herein.

[0016] Coherent Raman spectroscopy is of two types—scanned coherentRaman spectroscopy and multiplex coherent Raman spectroscopy. Scannedcoherent Raman spectroscopy uses narrowband tunable lasers. This methodgradually changes the frequency of one or more laser beams aimed at asample, while using equipment to monitor the size of a coherent Ramanbeam produced by the sample when irradiated by the frequency-changinglaser beams. But with this approach, the length of time required togenerate a single spectrum is long. In contrast, multiplex coherentRaman spectroscopy, which uses a combination of narrowband and broadbandlasers, allows data to be generated very quickly (in as little as one ora few laser pulses). In the past, a primary limitation of the multiplextechnique has been that the bandwidth of the lasers has not beensuitable to cover the entire vibrational spectrum with high spectralresolution. As a result, it has not been possible to achieve highlyaccurate identification of all samples, since the entire vibrationalRaman spectrum could not be produced with high spectral resolution.

[0017] Thus, there is a need for a multiplex coherent Raman spectrometerand multiplex coherent Raman spectroscopy method that can rapidlyproduce a vibrational Raman spectrum of a sample so that it covers theentire vibrational region with high spectral resolution, therebyimproving the accuracy of sample identification. More specifically,there is a need for a multiplex coherent Raman spectrometer andspectroscopy method that can rapidly produce the entire vibrationalRaman spectrum of approximately 3000 wavenumbers with sub-wavenumberresolution, thereby permitting highly accurate sample identification.

SUMMARY OF THE INVENTION

[0018] It is an object of the present invention to provide a multiplexcoherent Raman spectroscopy detector and method that can produce avibrational Raman spectrum of a sample covering more than 1000wavenumbers, thereby increasing the accuracy of sample identification.

[0019] It is a further object of the present invention to illuminate asample with broadband illumination of sufficient bandwidth that thesample will scatter coherent Raman light to produce a gas phasevibrational Raman spectrum of a sample covering more than 1000wavenumbers, and preferably at least 3000 wavenumbers, therebyincreasing the accuracy of sample identification.

[0020] It is still another object of the present invention to increasethe intensity of backward-propagating, phase-conjugate, coherent Ramanradiation produced by a Raman cell, and preferably to increase theintensity to provide a sufficiently strong input beam for pumping ordriving an optical parametric oscillator to produce a substantiallystable output.

[0021] According to one aspect, the present invention that achieves atleast one of these objectives relates to a multiplex coherent Ramanspectrometer and spectroscopy method for rapidly detecting andidentifying individual components of a chemical mixture separated by aseparations technique, such as gas chromatography. The spectrometer andmethod increase the accuracy with which a variety of compounds areidentified because they comprise means, elements, and steps to produce agas phase vibrational Raman spectrum of an unknown sample gas of morethan 1000 wavenumbers, and preferably can do so rapidly (within one or afew laser pulses), with a high signal-to-noise ratio and without anygaps in the spectrum. Preferably, the spectrometer and method accuratelyidentify a variety of compounds because they comprise means, elements,and steps to produce the entire gas phase vibrational Raman spectrum ofan unknown sample gas covering at least 3000 wavenumbers.

[0022] According to another aspect, the present invention provides anelement, a step, or means that drive a broadband-beam-producing deviceto simultaneously illuminate the sample with a stable broadband beam ofmore than 1000 wavenumbers bandwidth, and preferably more than 3000wavenumbers bandwidth and with a narrowband beam of less than 1wavenumber bandwidth and preferably about 0.003 wavenumbers bandwidth ofsufficient intensity to produce a gas phase vibrational Raman spectrumof the sample of more than 1000 wavenumbers, and preferably more than3000 with a spectral resolution of less than 1 wavenumber and preferablyabout 0.003 wavenumbers rapidly (within one or a few laser pulses) witha high signal-to-noise ratio and without any gaps in the spectrum.

[0023] The element, means, and step for driving such a broadband-beamproducing device can comprise a hydrogen-filled Raman cell, tilted byless than 2.2 degrees with respect to an input beam entering thesample-filled Raman cell, to produce a high-intensity,backward-stimulated, coherent Raman beam of 683 nm. More generally, thiselement, means, or step will produce such a high-intensity,backward-stimulated, coherent Raman beam of 683 nm when thehydrogen-filled Raman cell is tilted with respect to an input beam up to(but not exceeding) the angle at which less than the entire input beamenters a hole in the hydrogen-filled Raman cell, so that the focal pointof the entire input beam in the hydrogen-filled Raman cell collides witha side wall on the inside of the hydrogen-filled Raman cell, as shown inFIG. 5. This 683 nm beam can be used to drive a broadband-beam producingdevice, such as a degenerate optical parametric oscillator to produce astable broadband beam of 1100-1700 nm that covers a continuous range of3200 wavenumbers. This broadband beam is then combined with a narrowbandbeam of 532 nm having a bandwidth of less than 1 wavenumber andpreferably about 0.003 wavenumbers and focused into a heated windowlesssample-filled Raman cell that receives gases (i.e. the sample) separatedby a gas chromatograph. When these gases are illuminated with thecombined broadband and narrowband beams, they emit coherent Ramanradiation. This Raman radiation is of sufficient intensity and bandwidththat when it is filtered and then sent to a monochromator withmultichannel detection, complete vibrational Raman spectra of at least3000 wavenumbers is produced from one or a few laser pulses, without anygaps in the vibrational Raman spectra and with a high signal-to-noiseratio.

[0024] Other objects and features of the present invention will becomemore apparent upon consideration of the following detailed descriptionof preferred embodiments taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a schematic view of a first preferred embodiment of theMultiplex Coherent Raman Radiation Detector.

[0026]FIG. 2 is a schematic view of the optical parametric oscillatorused in the Multiplex Coherent Raman Radiation Detector.

[0027]FIG. 3 is a schematic view of a second embodiment of the MultiplexCoherent Raman Radiation Detector.

[0028]FIG. 4A shows a contour plot of the data from a monochromatormeasuring the time-lapse sequence of Raman spectra of various gassesafter they are separated by a gas chromatograph. FIG. 4B shows a graphof data from a monochromator measuring the Raman spectrum of the samegasses as in FIG. 4A, but before they are separated by the gaschromatography. FIG. 4C shows a graph of the integrated signal from themonochromator (integrated over wavelength) as a function of time.

[0029]FIG. 5 is a schematic view of the hydrogen-filled Raman cell 20shown in FIG. 1 and the tilting of the cell 20 with respect to the inputbeam.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] As used in this application, the term “coherent light source”refers to both laser and a device for producing coherent light, such asan optical parametric oscillator.

First Preferred Embodiment

[0031] Introduction

[0032]FIG. 1 is a schematic view of a first preferred embodiment of themultiplex coherent Raman detector 10. The detector 10 detects the Ramanradiation emitted from an unknown gas and uses that Raman radiation todetermine the identity of the gas with higher accuracy than has beenpossible before. This advantageous effect occurs because the FIG. 1embodiment illuminates the unknown gas simultaneously with a broadbandlaser beam having a bandwidth of more than 3000 wavenumbers and anarrowband laser beam having a bandwidth of less than 1 wavenumber andpreferably approximately 0.003 wavenumbers. This broadband andnarrowband radiation, when scattered by an unknown gas, produces theentire gas phase vibrational Raman spectrum with a spectral resolutionof less than 1 wavenumber and preferably approximately 0.003 wavenumberswithout any gaps therein, which permits an accurate identification ofthe gas. As a result, the range-to-resolution ratio of this system,which provides a measure of the ability of the system to distinguishbetween similar but not identical chemical species, is3000/0.003=1,000,000. The narrowband laser beam is produced by aninjection-seeded, near transform-limited Nd:YAG laser 12. The broadbandillumination is produced by an optical parametric oscillator 28. Theoptical parametric oscillator must be pumped by a sufficiently strong683 nm beam to generate stable broadband radiation of sufficientintensity that the Raman radiation scattered by the unknown gas can bedetected by a monochromator and a CCD. This task is accomplished byslightly tilting a hydrogen-filled Raman cell with respect to anincident 532 nm laser beam. Such tilting increases the intensity of the683 nm laser beam produced by the Raman cell to the required degree.

[0033] Production of broadband and narrowband light for sampleillumination Producing and splitting of laser beam

[0034] The detector 10 comprises an injection seeded Nd:YAG laser 12(manufactured by Spectraphysics, model no. GCR-230 laser with an EEO-355option) that produces a second harmonic beam whose wavelength is 532 nm.This type of laser is a Q-switched nanosecond laser and because it isinjection seeded, it produces a laser beam having a bandwidth of about0.003 wavenumbers. The laser 12 has a 10 Hz repetition rate and thesecond harmonic beam produced thereby has an energy of about 200 mJ per5 ns pulse. The second harmonic 532 nm beam is projected by the laser 12along optical axis 01 to a wedge reflector 14. The wedge reflector 14reflects a portion of the 532 nm beam (approximately 5 mJ) to a varietyof optical elements (not shown) including a Pellin Broca prism thatpurify it and delay its arrival at a gas chromatograph, as will bediscussed below. The remainder of the 532 nm beam passes through thewedge reflector 14 and is reflected by a dichroic mirror 16(manufactured by CVI, model no. Y2-1025-45-UNP). The dichroic mirror 16reflects 532 nm light and transmits 683 nm light. The dichroic mirror 16reflects the 532 nm beam along optical axis 02 to a 0.5 m focal-length,plano-convex lens 18 (manufactured by Coherent, model no. 43-0546-000)that focuses the beam along optical axis 02 into a cylindrical,stainless steel Raman cell 20 filled with hydrogen. In response toreceiving the focused 532 nm beam, the hydrogen-filled Raman cell 20produces a backward-propagating, phase-conjugate, stimulatedRaman-scattered 683 nm beam traveling back along the optical axis 02toward the lens 18.

[0035] Producing a broadband-light-production driving beam with a Ramancell

[0036] The hydrogen-filled cylindrical Raman cell 20 is one meter inlength, one inch in outer diameter, 0.75 inches in inner diameter, andis tilted approximately one degree with respect to the optical axis 02.Applicants have discovered that the tilting of the Raman cell increasesthe energy of the coherent Raman radiation by as much as four times ascompared to the energy of the coherent Raman radiation produced by thehydrogen-filled Raman cell 20 without tilting. Thus, in this embodiment,the energy of the 683 nm beam produced by the hydrogen-filled Raman cell20 can reach 130 mJ when the incident 532 nm beam is 340 mJ. Moretypically, the laser 12 produces a 532 nm beam whose energy whenentering the hydrogen-filled Raman cell 20 is approximately 200 mJ,thereby producing a 683 nm beam of 70 mJ. This increased energy of thecoherent Raman radiation produced by the hydrogen-filled Raman cell 20is needed to generate a sufficiently strong broadband beam in theoptical parametric oscillator (to be discussed below) that the Ramanradiation produced in response to illuminating an unknown gas with thebroadband beam has sufficient strength to be detected with a highsignal-to-noise ratio when using a monochromator and a CCD (chargecoupled device) and a computer. Without tilting, saturation of theRaman-scattering process within the hydrogen-filled Raman cell 20 limitsthe energy of the 683 nm pulse to about 30 mJ. In fact, at high 532 nmbeam energies, saturation can cause the 683 nm output beam energy toslightly decrease as the 532 nm input beam energy is increased.

[0037] Advantageously, the quality of the 683 nm phase-conjugate beamproduced by the hydrogen-filled Raman cell 20 is almost identical tothat of the 532 nm beam, which is high due to the Gaussian optics usedin the Nd:YAG 12. A high quality beam is a beam of a substantiallycircular cross-section of substantially uniform intensity throughout thearea of the cross-section.

[0038] Disadvantageously, the length of the backward-propagating pulseof the 683 nm beam produced by the hydrogen-filled Raman cell 20 is muchshorter than the pulse of the incident 532 nm beam. But thisdisadvantage is overcome in the present embodiment by creating a pulsetrain, using gas under high pressure in the hydrogen-filled Raman cell20, such as at least 200 psi and preferably 400 psi of hydrogen, and byusing an optical parametric oscillator of short cavity length of 5inches or less, as will be discussed below. More generally, the lengthof the OPO 28 is such that the time required for the 683 nm beam totravel twice the cavity length of the OPO 28 is less than the pulselength of the 683 nm beam.

[0039] The hydrogen-filled Raman cell 20 projects the 683 nm phaseconjugate beam along the optical axis 02 through the lens 18, whichcollimates and directs the beam through the dichroic mirror 16 to atelescope comprising a +175 mm focal-length, plano-convex lens 22(manufactured by Edmund Scientific, model no. 32,866) and a—100 mmfocal-length lens 24 (manufactured by Edmund Scientific, model no.45,027), which together reduce the diameter of the 683 nm beam. Thereduced-diameter 683 nm beam then travels along the optical axis 02 to ahalf-wave plate 26, which rotates the beam polarization to a verticalpolarization before it enters a free-running optical parametricoscillator (OPO) 28. In response to receiving the 683 nmvertically-polarized beam, the OPO 28 produces a broadband beam of1100-1700 nm covering a continuous range of over 3000 wavenumbers, andmore specifically 3200 wavenumbers. Using a beam with this broad abandwidth (in combination with a narrowband beam, as will be discussedbelow) permits the entire gas phase vibrational Raman spectrum of anunknown sample to be obtained rapidly, within 1 or a few laser pulses,without gaps and with a high signal-to-noise ratio, thereby permittingaccurate identification of the sample.

[0040] Producing broadband illumination with an optical parametricoscillator

[0041]FIG. 2 shows the OPO 28. The OPO 28 comprises a window 52receiving the reduced-diameter, vertically-polarized 683 nm beam fromthe half- wave plate 26. The OPO 28 also comprises a dichroic mirror 54(manufactured by CVI, model no. TLM1-690-0-1037) for reflecting the 683nm beam received by the window 52 to a mirror 56 (manufactured by CVI,model no. R1-1025-45-UNP). The mirror 56 reflects the 683 nm beam to anidentical dichroic mirror 58 that reflects the 683 nm beam through twoBBO (beta barium borate) crystals 60 and 62 cut for type I phasematching and that can be manually tilted. Each BBO crystal ismanufactured by Casix, is 5 mm×5 mm×14 mm and cut at 22 degrees, and hastwo opposed, small faces that are coated with a single layer ofmagnesium fluoride, which is centered at 700 nm. The two crystals 60 and62 are manually tilted so that they are tilted at similar angles, but inopposite directions and their angles are manually adjusted until theycontinuously emit broadband light in the range from 1100 nm to 1700 nmin response to receiving the 683 nm beam.

[0042] As a result, once the 683 nm light has initially passed throughthe crystals 60 and 62, two different types of light are projected fromcrystals 60 and 62—the 683 nm input beam and the 1100 nm-1700 nm outputbroadband beam. Both of these beams strike a dichroic mirror 64,identical to mirrors 56 and 58, which reflects the 683 nm beam andtransmits the 1100 nm-1700 nm broadband beam.

[0043] The dichroic mirror 64 reflects the 683 nm beam to a mirror 66identical to mirror 54, which, in turn, reflects the 683 nm beam back tothe dichroic mirror 64 and out of the OPO 28 through the window 52 viathe same path that this beam traveled through the OPO 28, i.e. backthrough the two crystals 60 and 62 to the dichroic mirror 58, whichreflects the 683 nm beam to mirror 56, which, in turn, reflects the beamto mirror 54, where the 683 nm beam is reflected to window 52.

[0044] In contrast, the 1100 nm-1700 nm broadband beam, which istransmitted through the dichroic mirror 64, strikes apartially-reflecting mirror 68 (manufactured by CVI, model no.PR2-1350-1600-20-1037 or PR1-1319-20-1037) that functions as an outputcoupler. The mirror 68 permits 80% of the 1100 nm-1700 nm broadband beamto pass therethrough and out of the OPO 28 through a window 72. Themirror 68 also reflects 20% of the 1100 nm-1700 nm broadband beam backto the dichroic mirror 64, which transmits the 1100 nm-1700 nm broadbandbeam back through the two crystals 60 and 62 and then through thedichroic mirror 58 to a mirror 70 (manufactured by Newport, model no.10D20ER.1). The mirror 70 reflects 100% of the 1100 nm-1700 nm broadbandbeam back through the dichroic mirror 58 and again through the twocrystals 60 and 62 and the dichroic mirror 64 to the mirror 68, whichagain transmits 80% of the 1100 nm-1700 nm broadband beam out of the OPO28 through the window 72 and reflects 20% of the 1100 nm-1700 nmbroadband beam back through the crystals 60 and 62 toward the dichroicmirror 58 and the mirror 70, as noted above, where the process iscontinuously repeated, until the beam output from window 72 in responseto a given pulse from the laser 12 diminishes to zero intensity.

[0045] In addition, the 1100 nm-1700 nm broadband beam is againgenerated anew as the 683 nm beam is reflected by the dichroic mirror 64through the two crystals 60 and 62. In this case, the newly generated1100 nm-1700 nm broadband beam is transmitted through the dichroicmirror 58 to the mirror 70, which reflects 100% of the 1100 nm-1700 nmbroadband beam back through the dichroic mirror 58 and again through thetwo crystals 60 and 62 and the dichroic mirror 64 to the mirror 68,which transmits 80% of the 1100 nm-1700 nm broadband beam out of the OPO28 through the window 72 and reflects 20% of the 1100 nm-1700 nmbroadband beam back through the crystals 60 and 62 toward the dichroicmirror 58 and the mirror 70, where the process is continuously repeated,until the beam output from window 72 in response to a given pulse fromthe laser 12 diminishes to zero intensity.

[0046] As a result of this process, a 1100 nm-1700 nm broadband beam isoutput from the window 72 at 5-10 mJ per pulse.

[0047] The OPO 28 has an intensity threshold of 10-30 mJ per pulse,below which a 683 nm beam input into the OPO will not produce a 1100nm-1700 broadband beam. The cell 20, when tilted 1 degree and inresponse to receiving a 200 mJ, 532 nm beam, produces a 683 nm beam ofan intensity above this threshold of the OPO 28. But, if the OPO 28 isdriven by an input 683 nm beam merely at this intensity threshold, itwill produce an unstable 1100-1700 nm output beam. To produce a stable1100-1700 nm output beam, the 683 nm input beam must be above thisthreshold by a substantial amount, for example, 50-100% above thethreshold. When the cell 20 is tilted 1 degree in response to receivinga 200 mJ,532 nm beam, the cell 20 produces a 683 nm beam whose intensityis above this threshold by the predetermined amount so as the drive theOPO 28 to produce a stable 1100 nm-1700 nm broadband beam.

[0048] Combining of broadband and narrowband beams for sampleillumination

[0049] Referring again to FIG. 1, the 1100 nm-1700 nm broadband beamemerging from the window 72 of the OPO 28 enters a filter 30(manufactured by Schott Glass Technologies, model no. RG830), whichfilters out any visible ambient light. The 1100 nm-1700 nm broadbandbeam then enters a 0.5 m focal-length, plano-convex lens 32(manufactured by Coherent, model no. 43-0546-000). The lens 32collimates the broadband beam, which then strikes a dichroic mirror 34(manufactured by CVI, model no. LWP-0-RUNP532-TUNP1000-2000). Thedichroic mirror 34 permits the 1100 nm-1700 nm broadband beam to passtherethrough, since it transmits light of wavelengths between 1000 nmand 2000 nm. In addition, the dichroic mirror 34 receives the 532 nmbeam from the laser 12 that has been reflected by the wedge reflector 14and delayed (by optical elements that are not shown in FIG. 1) so as toarrive at the dichroic mirror 34 at the same time as the 1100 nm-1700 nmbroadband beam collimated by the lens 32. The dichroic mirror 34reflects the delayed 532 nm beam along the optical axis 02 at the sametime that the 1100 nm-1700 nm broadband beam passes through the dichroicmirror 34 and also travels along optical axis 02. As a result, the 532nm beam and the 1100 nm-1700 nm broadband beam are spatially andtemporally overlapped as they travel from the dichroic mirror 34 alongoptical axis 02.

[0050] In an alternative embodiment of FIG. 1, the narrowband coherentbeam that is combined with the broadband coherent beam at the dichroicmirror 34, originates with a second OPO (not shown) rather than with thelaser 12. As a result, the reflected beam from the wedge reflector 14 isterminated by a beam block (not shown). In this alternative embodiment,there is provided a second OPO, having a tuning range from 220 nm to1800 nm, which is driven by a second laser (not shown). The second OPOgenerates a narrowband coherent beam having a bandwidth of 0.2wavenumbers. Optical elements (not shown) direct the narrowband coherentbeam from the second OPO to the dichroic mirror 34, which is differentfrom the mirror 34 in FIG. 1 in that it is designed to reflect thenarrowband coherent beam from the second OPO and combines thisnarrowband coherent beam from the second OPO with the broadband coherentbeam transmitted through the lens 32 so as to spatially and temporallyoverlap these two beams as they travel from the mirror 34 along opticalaxis 02. In this alternative embodiment, all the other elements are thesame as in FIG. 1.

[0051] In both the embodiment shown in FIG. 1, and the alternativeembodiment of FIG. 1, the combined beams will be used to illuminate anunknown sample gas to create a multiplex coherent Raman beam. The methodof creating a multiplex coherent Raman beam using a single narrowbandlaser beam and broadband light resembles a technique known as dualbroadband CARS that uses two broadband dye lasers, a method developed byEckbreth and Anderson, in “Dual broadband CARS for simultaneous,multiple species measurements”, Applied Optics vol. 24, 1985, 2731-2736.Here, a single broadband OPO takes the place of the two broadband dyelasers used by Eckbreth and Anderson. In addition, here the signal isdetected over a range of 450-530 nm. The use of this approach permitshigh resolution spectra to be generated without gaps, which may not bethe case using other multiplex methods.

[0052] Illuminating an unknown sample with narrowband and broadbandlight

[0053] The two overlapping beams then pass through an 8 inchfocal-length, plano-convex lens 36 (manufactured by Edmund Scientific,model no. 45,152), which focuses the two overlapping beams at a point onoptical axis 02 in a heated sample-filled Raman cell 38 that receives astream of gases (i.e. the unknown sample whose identity is to bedetermined by detector 10) from a gas chromatograph 40 (manufactured byGow Mac, model no. 400). The two overlapping beams are then scattered bythe unknown sample gas to produce coherent Raman radiation, whosespectrum will be detected to determine the identity of the gas. Lens 32ensures that the lens 36 focuses the two overlapping beams at the samepoint on optical axis 02. This use of overlapping narrowband andbroadband beams to illuminate a sample to produce spectrally broadcoherent Raman radiation is known as multiplex coherent Raman scatteringand is necessary to produce coherent Raman spectra rapidly (at 10 Hz,the repetition rate of laser 12). By this arrangement, a Raman spectrumcan be produced during one or two laser pulses. And since the repetitionrate of the laser 12 is 10 Hz, a new Raman spectrum can be producedevery {fraction (1/10)} of a second, the time between laser pulses. Inaddition, because the broadband beam is over 3000 wavenumbers inbandwidth, when this beam is scattered by the unknown gas to produce acoherent Raman beam, the coherent Raman beam will also be over 3000wavenumbers in bandwidth, thereby producing the entire gas phasevibrational Raman spectrum of the unknown gas without any gaps, whichpermits accurate identification of the gas.

[0054] The sample-filled Raman cell 38 is composed of a piece of coppertubing, attached at one end to the output of the gas chromatograph 40,and a T-shaped, hollow brass joint, whose bottom end (the bottom of the“T”) is attached to the other end of the copper tubing. The coppertubing and T-shaped brass joint are wrapped in heating tape. The twosides at the top of the “T” are open at each end to permit light tofreely enter and exit therefrom. The cell has no windows and is heatedto prevent condensation of the sample gases as they emerge from the gaschromatograph 40.

[0055] Optically and electrically processing the coherent Raman beamfrom the sample

[0056] The overlapping beams, called the input beams, interact with theflowing gases in the sample-filled Raman cell 38 to generate a coherentRaman beam. The coherent Raman beam, called the output beam, exits fromone side of the “T” opposite from where the overlapping beams enteredthe “T”. The output coherent Raman beam, which is mixed with the input,overlapping beams, is then reshaped and collimated by an 8 inchfocal-length, plano-convex lens 42 (manufactured by Edmund Scientific,model no. 45,152), and enters optical filters 44 and 46 (manufactured bySchott Glass Technologies nos. KG3 and BG40, respectively), which absorbnear infra-red light to remove one of the input beams. The remaining 532nm input beam and the coherent Raman beam strike a holographic notchfilter 47 (manufactured by Kaiser Optical Systems, model no.HNF-532.0-1.0), which removes the 532 nm beam by reflection. In the caseof the alternative embodiment of FIG. 1 where the 532 nm beam isreplaced by a narrowband coherent beam from a second OPO, filter 47 isreplaced by a holographic notch filter made for the wavelength of thebeam from the second OPO (such as model no. HNF-633-1.0 or HNF-488-1.0manufactured by Kaiser Optical systems). The remaining output beam isthen focused by an 8 inch focal-length, plano-convex lens 48(manufactured by Edmund Scientific, model no. 45,152) into a 1.25 mmonochromator 50 (manufactured by SPEX, model 1250 m) that is equippedwith a 150 g/mm grating and a charge-coupled device (CCD) 51. The CCD 51records spectra at a rate of 10 Hz (the repetition rate of the laser12). The CCD 51 is attached to a computer (not shown) running softwareto analyze and display the data produced by the CCD 51. The computerstores the Raman spectra of the unknown gas and because the Ramanspectrum covers a region of more than 3000 wavenumbers, the resultingspectra can be used to identify the gas from its Raman spectrum withhigh accuracy, as will be discussed in further detail below inconjunction with FIGS. 4A, 4B, and 4C. The computer may also average oraccumulate the spectra produced by the CCD over longer time periods toimprove signal-to-noise ratios and reduce the number of data files. Thecomputer may, in addition, permit the data to be viewed as a sequence ofspectra on a cathode ray tube or on a printed graph, as shown in FIG.4A, or in matrix form (intensity as a function of time and wavelength).

Second Preferred Embodiment

[0057]FIG. 3 shows a second preferred embodiment of the multiplexcoherent Raman detector 100. The laser, the OPO, the hydrogen-filledRaman cell, the sample-filled Raman cell, and the gas chromatograph arethe same in the two embodiments, as are a number of the opticalelements, as will be described below.

[0058] Introduction

[0059] The detector 100 detects the Raman radiation scattered from anunknown gas and uses that Raman radiation to determine the identity ofthe gas with higher accuracy than has been possible before. Thisadvantageous effect occurs because the FIG. 3 embodiment alsoilluminates the unknown gas simultaneously with a broadband laser beamhaving a bandwidth of more than 3000 wavenumbers and a narrowband laserbeam having a bandwidth of approximately 0.003 wavenumbers. Thisbroadband and narrowband radiation, when scattered by an unknown gas,produces the entire gas phase vibrational Raman spectrum with a spectralresolution of approximately 0.003 wavenumbers without gaps and with ahigh signal-to-noise ratio, which permits an accurate identification ofthe gas. The narrowband laser beam is produced by an injection-seeded,near transform-limited Nd:YAG laser 12, which produces a laser beamhaving a bandwidth of 0.003 wavenumbers due to its being injectionseeded. This broadband illumination is produced by an optical parametricoscillator. The optical parametric oscillator must be pumped by asufficiently strong 683 nm beam to generate stable broadband radiationof sufficient intensity that the Raman radiation scattered by theunknown gas can be detected by a monochromator and a CCD. This task isaccomplished by slightly tilting a hydrogen-filled Raman cell withrespect to an incident 532 nm laser beam and optical axis. Such tiltingincreases the intensity of the 683 nm laser beam produced by the Ramancell to the required degree.

[0060] Production of broadband and narrowband light for sampleillumination Producing and splitting of laser beam

[0061] The detector 100 comprises a 3 foot by 12 foot optical table 102supporting the components of the detector 100. The detector 100 furthercomprises an injection seeded Nd:YAG laser 104 that produces a secondharmonic beam whose wavelength is 532 nm. The laser 104 has a 10 Hzrepetition rate and the second harmonic beam produced thereby has anenergy of about 200 mJ per 5 ns pulse. The second harmonic 532 nm beamis projected by the laser 104 to a dichroic mirror 106 that reflects 532nm light and transmits 1064 nm light. 1064 nm light from the laser 104,which is not used in the detector 100, is transmitted through thedichroic mirror 106 to a beam dump 108. 532 nm light from the laser 104is reflected by the mirror 106 to a dichroic mirror 110, which reflectsthe 532 nm beam to a dichroic mirror 112. Mirror 110 is identical tomirror 106. Mirror 112 reflects the 532 nm beam to a wedged window 114.

[0062] Splitting of the laser beam and optically delaying one of thesplit beams

[0063] The wedged window 114 reflects a portion of the 532 nm beam(approximately 5 mJ) to a variety of optical elements that delay itsarrival at a gas chromatograph (identical to the gas chromatograph 40used in the FIG. 1 embodiment). More specifically, the wedged window 114reflects a portion of the 532 nm beam to a Pellin Broca prism 116, whichreflects and refracts the beam through an iris diaphragm 118 to abroadband dielectric mirror 120. Prism 116 directs 532 nm light towardthe diaphragm 118 and separates this 532 nm light from other wavelengthsby refracting other wavelengths at a different angle than the 532 nmlight. Therefore, diaphragm 118 spatially prevents wavelengths of lightthat are other than 532 nm from passing therethrough. The mirror 120reflects the 532 nm beam to a silver coated mirror 122, which, in turn,reflects the 532 nm beam to a dichroic mirror, to be discussed later,that reflects the 532 nm beam, while transmitting a broadband 1100-1700nm beam therethrough, thereby combining the beams for multiplexillumination of an unknown sample gas separated in the gaschromatograph, as will also be discussed later.

[0064] Transmitting of other split laser beam to ee cell

[0065] The remainder of the 532 nm beam that has not been reflected bythe wedged window 114 passes through the wedged window and is reflectedby a dichroic mirror 124. The dichroic mirror 124 reflects the 532 nmbeam to a dichroic mirror 126, which in turn, reflects the 532 nm beamto a dichroic mirror 128. The dichroic mirror 128 reflects 532 nm light,while transmitting 683 nm light. As a result, the dichroic mirror 128reflects the 532 nm beam along optical axis 01 to a 0.5 m focal-lengthplano-convex lens 130 that focuses the beam along optical axis 01 into astainless steel Raman cell 132 filled with hydrogen. In response toreceiving the focused 532 nm beam, the hydrogen-filled Raman cell 132produces a backward-propagating, phase-conjugate, stimulatedRaman-scattered 683 nm beam traveling back along the optical axis 01toward the lens 130. The hydrogen-filled Raman cell 132 also producesradiation of other wavelengths, which is collected in a beam dump 134.

[0066] Producing a broadband-light-production driving beam with a Ramancell

[0067] The hydrogen-filled Raman cell 132 is one meter in length and istilted approximately one degree with respect to the optical axis 01, theaxis along which the input 532 nm beam travels. Applicants havediscovered that the tilting of the hydrogen-filled Raman cell 132increases the energy of the coherent Raman radiation it produces by asmuch as four times as compared to the energy of the coherent Ramanradiation produced by the hydrogen-filled Raman cell 132 withouttilting. Thus, in this embodiment, the energy of the 683 nm beamproduced by the hydrogen-filled Raman cell 132 can reach 130 mJ when theincident 532 nm beam is 340 mJ. More typically, the laser 104 produces a532 nm beam whose energy when entering the hydrogen-filled Raman cell132 is approximately 200 mJ, thereby producing a 683 nm beam of 70 mJ.This increased energy of the coherent Raman radiation produced by thehydrogen-filled Raman cell 132 is needed to generate a sufficientlystrong and stable broadband beam in the optical parametric oscillator(to be discussed below) that the Raman radiation produced in response toilluminating an unknown gas with the broadband beam has sufficientstrength to be detected with a high signal-to-noise ratio without anygaps when using a monochromator, a CCD and a computer to determine theidentity of the unknown sample gas. Without tilting, saturation of theRaman-scattering process within the hydrogen-filled Raman cell 132limits the energy of the 683 nm pulse to about 30 mJ. In fact, at high532 nm beam energies, saturation can cause the 683 nm beam energy toslightly decrease as the 532 nm beam energy is increased.

[0068] In addition, the effect of tilting was studied by focusing a beamof light from a tunable OPO into the Raman cell. The energy of thefocused beam from the OPO remained a constant value of 52 mJ over thewavelength range of 531.7 nm to 532.3 nm. When not tilted, thebackward-propagating stimulated Raman-scattered beam generated by theRaman cell was measured to be 13+/−2 mJ. After tilting, thebackward-propagating beam energy increased to 18+/−2 mJ over the samewavelength range. The size of the increase was limited by the fact thatthe energy from the tunable OPO was small compared to that of Nd:YAGlaser 104. The fact that the increase was observed over a range ofwavelengths suggests that the effect responsible for the increase isinsensitive to wavelength.

[0069]FIG. 5 shows the tilting of the hydrogen-filled Raman cell 132with respect to the optical axis 01 and the input beam, focused by thelens 130 on an inside side wall of the cell 132, after passing throughthe window of fused silica on the front face of the cell 132. The cell132 is tilted, for maximum 683 nm beam energy, up to the angle at whichless than the entire input beam enters a hole in the hydrogen-filledRaman cell, so that the focal point of the entire input beam in thehydrogen-filled Raman cell 132 collides with a side wall on the insideof the hydrogen-filled Raman cell. The structure of the cell 132 in FIG.5 is identical to the cell 20 shown in FIG. 1 and cell 20 is tilted inan identical manner to the tiling of cell 132 in FIGS. 3 and 5.

[0070] As in the first embodiment, the quality of the 683 nmphase-conjugate beam produced by the Raman cell 132 is high and isalmost identical to that of the 532 nm beam, which is high due to theGaussian optics used in the Nd:YAG laser 104. In addition, the length ofthe backward-propagating pulse of the 683 nm beam produced by the Ramancell 132 is much shorter than the pulse of the incident 532 nm beam. Butthis disadvantage is overcome, as in the first embodiment, by creating apulse train, using gas under high pressure in the Raman cell 132, suchas at least 200 psi and preferably 400 psi of hydrogen, and by using anoptical parametric oscillator of short cavity length of 5 inches orless, as will be discussed below.

[0071] The Raman cell 132 projects the 683 nm phase conjugate beam alongthe optical axis 01 through the lens 130, which collimates the beam anddirects it through the dichroic mirror 128 to a telescope comprising a+175 mm focal-length, a plano-convex lens 136 and a −100 mmfocal-length, plano-concave lens 138, which together reduce the diameterof the 683 nm beam. The reduced-diameter 683 nm beam then travels alongthe optical axis 01 to a half-wave plate 140, which rotates the beampolarization to a vertical polarization before it enters a free-runningoptical parametric oscillator (OPO) 142.

[0072] Producing broadband illumination with an optical parametricoscillator

[0073] In response to receiving the 683 nm vertically-polarized beam,the OPO 142 produces a broadband beam of 1100-1700 nm covering a rangeof over 3000 wavenumbers, and more specifically 3200 wavenumbers. Usinga beam with this broad a bandwidth (in combination with a narrowbandbeam, as will be discussed below) permits the entire gas phasevibrational Raman spectrum of an unknown sample to be obtained, therebypermitting accurate identification of the sample. The OPO 142 is thesame as the OPO 28 shown in FIGS. 1 and 2. Therefore, no furtherdiscussion of this element is provided.

[0074] The OPO 142 outputs a 1100 nm-1700 nm broadband beam at 5-10 mJper pulse of the laser 104.

[0075] Calibrating the OPO before sample illumination

[0076] The 1100 nm-1700 nm broadband beam emerging from the window ofthe OPO 142 enters a filter 144, which removes out any visible ambientlight by absorption. The 1100 nm-1700 nm broadband beam then enters a0.5 m focal-length piano-convex lens 146, which collimates the broadbandbeam. The collimated broadband beam then strikes a removable right angleturning prism 148. The prism 148 is placed on optical axis 01 beforemeasurement of the Raman spectrum of a sample from a gas chromatographfor the purpose of determining the spectrum of the broadband beamgenerated by the OPO 142, as will be discussed below. The prism 148 isremoved after such measurement and before the measurement of the Ramanspectrum of the sample from the gas chromatograph. When the prism 148 ispositioned at the position on the optical axis 01 shown in FIG. 3, the1100-1700 nm beam from the OPO 142 is reflected to an identical rightangle turning prism 150. The prism 150 reflects the beam through a 0.75m focal-length, plano-convex lens 151 to the entrance of a 0.125 mscanning monochromator 152 with a pyroelectric detector, such as aMolectron Joulemeter (model J9LP). The signal from the pyroelectricdetector is transmitted to a preamplifier (not shown) and then to agated boxcar integrator (not shown), such as a Stanford Research Systemsboxcar integrator, which captures the signal electronically so that itcan be recorded. The resulting signal is recorded using a computer (notshown), equipped, for example with a data acquisition system and Labviewsoftware from National Instruments, which permits the spectrum of theOPO 142 to be determined before Raman-spectrum measurement of theunknown gas sample generated by the gas chromatograph.

[0077] Combining of broadband and narrowband beams for sampleillumination

[0078] When the prism 148 is removed from the optical axis 01, the1100-1700 nm beam from the lens 146 strikes the dichroic mirror 154,which transmits the 1100-1700 nm broadband beam therethrough (it iscapable of transmitting light from 1000 nm to 2000 nm therethrough). Inaddition, the dichroic mirror 154 receives the 532 nm beam from themirror 122. The 532 nm beam has been delayed by wedged window 114, thePellin Broca prism 116, the broadband dielectric mirror 120, and themirror 122 so as to arrive at the dichroic mirror 154 at the same timeas the 1100 nm-1700 nm broadband beam from lens 146. The dichroic mirror154 reflects the delayed 532 nm beam along the optical axis 01 at thesame time that the 1100 nm-1700 nm broadband beam passes through thedichroic mirror 154 and also travels along optical axis 01. As a result,the 532 nm beam and the 1100 nm-1700 nm broadband beam are spatially andtemporally overlapped as they travel from the dichroic mirror 154 alongoptical axis 01. Alternatively, a separate laser or laser-like source(not shown) may be used as the source of the 532 nm beam. If thisalternative source generates a wavelength other than 532 nm, then adifferent dichroic mirror that reflects this new wavelength may be used.An example of an alternative source is an OPO such as a SpectraphysicsMOPO 730 with a frequency doubler, which can generate light over therange of 220 nm to 1800 nm.

[0079] Illuminating an unknown sample with narrowband and broadbandlight

[0080] The two overlapping beams then pass through an 8 inchfocal-length, plano-convex lens 156, which focuses the two overlappingbeams to a point on optical axis 01 in a heated, sample-filled Ramancell 158 that receives a stream of gases (i.e., the unknown sample whoseidentity is to be determined by detector 100) from a gas chromatograph160. The two overlapping beams interact with the unknown gas to producecoherent Raman radiation, whose spectrum can be analyzed to determinethe gas's identity, as will be discussed below. Lens 146 ensures thatthe lens 156 focuses the two overlapping beams at the same point onoptical axis 01. This use of overlapping narrowband and broadband beamsto illuminate a sample to produce spectrally broad coherent Ramanradiation is known as multiplex coherent Raman scattering and isnecessary to produce coherent Raman spectra rapidly (at 10 Hz, therepetition rate of laser 104). In addition, because the broadband beamis over 3000 wavenumbers in bandwidth, when this beam is scattered bythe unknown gas to produce a coherent Raman beam, the coherent Ramanbeam will also be over 3000 wavenumbers in bandwidth, thereby producingthe entire gas phase vibrational Raman spectrum of the unknown gaswithout gaps, which permits accurate identification of the gas. Thesample-filled Raman cell 158 is the same as the sample-filled Raman cell38 in the FIG. 1 embodiment.

[0081] The overlapping beams, called the input beams, enter one of thetwo sides at the top of the “T” and interact with the flowing gases inthe sample-filled Raman cell 158 to generate a coherent Raman beam. Thecoherent Raman beam, called the output beam, exits from the other of thetwo sides at the top of the “T” along with the input, overlapping beams.The output, coherent Raman beam, which is mixed with the input,overlapping beams, is then reshaped and collimated by an 8 inchfocal-length, plano-convex lens 162 and enters optical filters 164 and166, which absorb near infra-red light to remove one of the input beams.The remaining 532 nm input beam and the coherent Raman output beam thenstrike a silver coated mirror 168, which reflects these beams to abroadband dielectric mirror 170. The mirror 170 reflects these beamsthrough a holographic notch filter 172, which removes the 532 nm inputbeam by reflection.

[0082] Adjusting the detector 100 to maximize the coherent Raman beam

[0083] A removable silver coated mirror 174 is positioned in the opticalpath 01 of the filtered output beam, filtered by the filters 164, 166,and 172, before identification of the sample in the sample-filled Ramancell 158 for the purpose of adjusting the optics in the detector 100 tomaximize the intensity of the Raman radiation scattered from the samplegas in the sample-filled Raman cell 158. To accomplish this goal, themirror 174 is inserted into the optical path 01 and light from laser 104is projected through the optics of the detector 100 to the sample-filledRaman cell 158 when the sample-filled Raman cell 158 has only ambientair therein. Coherent Raman radiation generated from the ambient air andfiltered by the filters 164, 166, and 172 is reflected by the mirror 174to a removable silver coated mirror 176, which reflects the filteredoutput beam through a 2 inch focal-length, plano-convex lens 178, whichfocuses the filtered output beam into a 0.125 m double monochromator 180with an R928 photomultiplier tube (PMT). The PMT measures the intensityof the coherent Raman signal so that the optics of the detector 100 canbe adjusted before sample identification to maximize the intensity ofthe coherent Raman signal. This is accomplished by setting themonochromator 180 to a wavelength of a species that is present inambient air, such as nitrogen, which is 473 nm. The signal is thenviewed on an oscilloscope and the optics that direct the 532 nm beam andthe OPO beam into the sample-filled Raman cell 158 are adjusted tomaximize the signal detected by the PMT.

[0084] Processing of coherent Raman radiation

[0085] Once the intensity of the coherent Raman radiation has beenoptimized, either 1) the mirror 176 is removed from the optical path ofthe filtered output beam reflected by the mirror 174, so that thefiltered output beam reflected by mirror 174 strikes a silver coatedmirror 182, which reflects the filtered output beam through a 50 mmfocal-length, plano-convex lens 184, which focuses the filtered outputbeam into a 0.25 m monochromator 186 with an Andor Technology iCCD (notshown), or 2) the mirror 174 is removed from the optical path 01, sothat the filtered output beam having passed through filter 172 isreflected by a broadband dielectric mirror 188 to a silver coated mirror190, which reflects the filtered output beam through an 8 inchfocal-length plano-convex lens 192, which focuses the filtered outputbeam onto a 1.25 m monochromator 194 with a CCD (not shown).

[0086] The CCD and the iCCD are connected to computers (not shown)running SpectraMax for Windows software or Intraspec software to controlthe monochromators, and to acquire data from the CCD and the iCCD and todisplay and analyze the results. The computer records and analyzes theRaman spectrum of the unknown gas and because the Raman spectrum hasmore than 3000 wavenumbers, the spectrum can be used to identify the gasfrom its Raman spectrum with high accuracy, as will be discussed infurther detail below in conjunction with FIGS. 4A, 4B, and 4C. Forspecies with similar spectra, the ability to distinguish between themoften depends upon the resolution of the instrument. For additionalaccuracy in identifying such species, the monochromators may be operatedin a high resolution mode by using high density gratings (1200 g/mm or2400 g/mm). Other ways to improve the resolution include decreasing thesize of the pixels on the CCD and increasing the focal length of themonochromator. For example, monochromator 194 has a focal length of 1.25meters, a CCD pixel size of 27 microns, and can be equipped with a 1200g/mm grating. The resulting pixel-to-pixel resolution is 0.7 wavenumbers(in units of cm⁻), which is considerably better than the resolutioncommonly found in infrared detectors for gas chromatography (4 or 8wavenumbers). The computer may also average or accumulate the spectraproduced by the CCD over longer time periods to improve signal-to-noiseratios and reduce the number of data files. The computer may, inaddition, permit the data to be viewed as a sequence of spectra on acathode ray tube or on a printed graph, as shown in FIG. 4A, or inmatrix form (intensity as a function of time and wavelength).

[0087] The FIG. 3 embodiment can identify a sample of mass of at least100 micrograms. But it is within the scope of the present invention tomodify the FIG. 3 embodiment to reduce this number by a factor of 104 byreducing the size of the sample-filled Raman cell 158 to have anhourglass shape (with its smallest diameter being of the order of tensof microns) to match the shape of the overlapping beams when they arefocused inside the sample-filled Raman cell 158. It is also within thescope of the present invention to further reduce the detection limits ofthe mass of the sample by a factor of 10-10⁶ by using resonanceenhancement by varying the wavelength of the 532 nm beam to match theelectronic level of the sample molecule. The limit on the mass of thesample that can be identified is determined ultimately by the level ofnoise in the nonresonant background, which is a nonzero broad backgroundthat is normally found in coherent (nonlinear) spectroscopy.

[0088] Below in Table 1 are listed the manufacturers and model numbersof many of the elements of the FIG. 3 embodiment. TABLE 1 ElementManufacturer Model Number 102 Newport RS 3000 104 Spectraphysics GCR-230with EEO-355 option 106 CVI BSR-51-1025 108 Spectraphysics BD5 110 CVIBSR-51-1025 112 CVI Y2-1025-45-UNP 116 CVI PLBC-10.0-79.5C 118 ThorlabsID25 120 Thorlabs BB1-r1 122 Newfocus 5103 124 CVI Y2-1025-45-UNP 126CVI Y2-1025-45-UNP 128 CVI Y2-1025-45-UNP 130 Coherent 43-0546-000 132Taitech 1 meter stainless steel cell 134 Thorlabs BT510 136 EdmundScientific 32,866 138 Edmund Scientific 45,027 140 LaserVision no modelnumber 142 LaserVision Custom-built, see FIG. 2 144 Schott Glass Tech-RG830 nologies 146 Coherent 43-0546-000 148 Coherent 24-8096-000 150Coherent 24-8096-000 151 Coherent 43-0553-000 152 CVI cm110 154 CVILWP-0-RUNP532-TUNP1000-2000 156 Edmund Scientific 45,152 160 Gow Mac 400162 Edmund Scientific 45,152 164 Schott Glass Tech- KG3 nologies 166Schott Glass Tech- BG40 nologies 168 Newfocus 5103 170 Thorlabs BB1-r1172 Kaiser Optical Systems HNF-532.0-1.0 174 Newfocus 5103 176 Newfocus5103 178 Edmund Scientific 32,971 180 CVI cm112 182 Newfocus 5103 184Edmund Scientific 32,971 186 Oriel MS257 188 Thorlabs BB1-r1 190Newfocus 5103 192 Edmund Scientific 45,152 194 Spex 1250 m

[0089] Table 2, shown below, lists the preferred values and preferredranges of some of the elements in the embodiments shown in FIGS. 1 and3. TABLE 2 Item Value Range Hydrogen Pressure in Raman cell 400 psi >200psi used to produce 683 nm beam Pump wavelength for Raman cell 532 nm500-550 nm used to produce 683 nm beam Tilt angle of Raman cell 1.0degrees >0 but <2.2 degrees, so that beam focuses onto side of Ramancell. 532 nm pump energy 200 mJ in a 50-400 mJ input into Raman cell 5-8ns pulse producing 683 nm beam Wavelength to pump OPO 683 nm 650-700 nmBBO crystal Type I BBO, 5 mm × 5 mm × 14 mm (long), two opposite smallfaces coated with a single layer of magnesium fluoride, centered at 700nm (broad- band), cut at 22 ± 0.5 degrees. Length of oscillator cavity 5inches 1-10 inches of OPO Length of Raman cell 1 meter 0.1-2 metersproducing 683 nm beam Telescope lenses Focal lengths of Any combinationthat +175 mm and −100 mm reduces the beams to match the aperture of theBBO crystals (5 mm × 5 mm). Repetition rate of laser 10 Hz Any rate thatis sufficiently high to take spectra rapidly (e.g., >1 Hz). Carrier gasfor gas Nitrogen Any carrier gas that chromatograp can be used for gaschromatography

[0090] Timing of various operations

[0091] In both preferred embodiments described above, the unknown sampleis injected into the gas chromatograph at the same time that dataacquisition is started by projecting a laser pulse out of the laser andrecording data with the computer. The light-producing components of thedetectors 10 and 100 are preferably turned on 30-45 minutes in advanceof the injection (e.g. the laser, the hydrogen-filled Raman cellproducing the 683 nm beam, and the. OPO) to ensure stability of thecomponents, and are usually left running before and between movement ofgas from the gas chromatographs 40 and 160 into the sample-filled Ramancells 38 and 158. The gas chromatographs 40 and 160 are preferablyturned on several hours before the injection of the sample and movementof the gaseous sample from the gas chromatographs 40 and 160 into thesample-filled Raman cells 38 and 158 to ensure temperature stability. Inaddition, carrier gases which carry the sample placed in the gaschromatograph can include nitrogen (which provides a single sharp peakat 472 nm), or helium, which has no vibrational Raman signal because itis an atom, not a molecule, and only molecules produce vibrational Ramanspectra. The peak from the nitrogen can be used as a method to calibratethe detectors 10 and 100, as has been previously discussed.

[0092] Experimental Results

[0093] The FIG. 3 embodiment has been used to detect and identify thecomponents of mixtures after they have been separated using gaschromatography. A mixture containing benzene, acetone, methanol, carbontetrachloride, chloroform, and cyclohexane, was separated with gaschromatography, and the individual components were identifiedspectroscopically, even when chromatographic peaks were overlapped, aswill now be discussed.

[0094]FIG. 4A shows a two-dimensional reduction of a three-dimensionalcontour plot generated by a computer from data produced by a CCDconnected to monochromator 194 with a 150 g/mm grating measuring thecoherent Raman radiation scattered from a gas mixture containing the sixcompounds that was injected in the gas chromatograph 160. The X axisrepresents wavelength in nanometers, ranging from 450 nm to 530 nm,while the Y axis represents time, ranging from 0 seconds to 128 seconds.The Z axis (not shown) of the three-dimensional contour plot representsthe intensity of the detected light. Intensity is depicted in thetwo-dimensional graph shown in FIG. 4A by the spacing between contourlines, i.e., the closer the lines, the greater the change in signal at aparticular time and wavelength. Signals that are more intense cause agreater change in the contour plot and appear as darker regions of moreclosely spaced lines. Over a period of 128 seconds, different gases,that have been separated in time by the gas chromatograph 160 will enterthe sample-filled Raman cell 158 for measurement. Therefore, over time,the detector 100 will detect coherent Raman radiation from differentcompounds. And since these different compounds produce (when illuminatedwith the overlapping narrowband and broadband light) coherent Ramanradiation having different spectra from each other, the coherent Ramanspectra shown in FIG. 4A will change over the course of 128 seconds,which is what is seen in FIG. 4A.

[0095]FIG. 4B is a graph of data generated using the same gas speciesand the same equipment used in FIG. 4A, except that a 1200 g/mm gratingwas used in conjunction with the monochromator 194, and except thatmeasurements of the Raman spectrum of the mixture were taken while thegas mixture was held stationary in a separate room-temperature cellbefore the gases of the mixture were separated by the gas chromatograph.Moreover, unlike FIG. 4A, the Y axis represents the intensity of theRaman radiation detected by the CCD and the computer. FIG. 4B containsseventeen numbers, nos. 1-17, each of which designate a different peakon the graph. Each peak is associated with a specific compound, becausedifferent compounds have characteristic peaks on such a graph of theirRaman spectrum. Moreover, different compounds will shift the wavelengthand wavenumbers of light projected thereon by a characteristic and knownamount when they scatter such light. Therefore, from the-various peakson such a graph, the identity of the compounds in the mixture can bedetermined. Accordingly, Table 3 below lists the identity of eachcompound, called a species, the wavelength at which its characteristicpeak exists, and the shift in wavenumbers produced when the compoundscatters light to produce coherent Raman radiation. TABLE 3 Peak SpeciesWavelength Shift in wavenumbers (cm⁻¹) 1 Benzene 457.3 3073 2 Chloroform458.2 3030 3 Cyclohexane 459.9 2950 4 Acetone 460.2 2936 5 Cyclohexane460.2 2936 6 Cyclohexane 461.8 2860. 7 Methanol 462.2 2841 8 Nitrogen473.2 2339 9 Oxygen 491.4 1556 10 Methanol 503.6 1063 11 Methanol 504.51027 12 Benzene 505.4 992 13 Cyclohexane 510.3 802 14 Acetone 511.5 75615 Chloroform 513.7 672 16 Carbon tetrachloride 519.5 455 17 Chloroform522.1 359

[0096] The identity of species in FIG. 4A can be determined by findingthe X axis position of the event (or peak, characterized by a darkregion of closely spaced lines) and comparing it to a list of positionsfor various compounds such as are listed in Table 3. If a compound fromTable 3 is present at any time during a measurement, there will be anevent at the corresponding X axis positions. Tables of compounds thatare very large are referred to as “libraries,” and can be used as areference for identifying unknown compounds. When used with theselibraries, computers can be used to match spectra from unknown compoundswith known compounds and identify the unknown compounds automatically.For example, nitrogen (peak #8 in FIG. 4B) is the carrier gas thatcarries the six-compound mixture in the gas chromatograph, and ispresent at all times during the separation in the gas chromatograph.This fact is confirmed by the presence of a dark solid vertical line onthe graph in FIG. 4A directly above peak #8 in FIG. 4B. Moreover, theportion of the graph in FIG. 4A above peak #1 (representing benzene) inFIG. 4B appears slightly before the midpoint (in time) of theexperiment, indicating that benzene appears at this point in time. Inaddition, peak #12 in FIG. 4B also represents benzene, and appears atthe same point in time as peak #1. The portion of the graph in FIG. 4Afor cyclohexane above peaks #3, #5, #6, and #13 in FIG. 4B appear aroundthe same time as those for benzene. Benzene and cyclohexane aretherefore not resolved temporally by the gas chromatograph itself, butare resolved spectroscopically because the peaks appear at differentwavelengths. On the other hand, peak #5 for cyclohexane is notspectroscopically resolved from peak #4 for acetone. However, acetoneappears in FIG. 4A earlier (lower) than cyclohexane (acetone's featuresin FIG. 4A are above peaks #4 and #14 in FIG. 4B) and is thereforetemporally resolved. Despite the fact that peaks #4 and #5 occur at thesame wavelength, cyclohexane and acetone can be distinguished (by notingthe arrival of features in FIG. 4A above peak #14 before features inFIG. 4A above peaks #3, #6, and #13) because the instrument's bandwidthis sufficient to cover the entire vibrational region. By using thisprocedure, all the compounds in the mixture injected into the gaschromatograph 160 can be determined with absolute certainty.

[0097]FIG. 4C shows a graph of the integrated signal (integrated overwavelength) from the CCD as a function of time. The Y axis is identicalto that of FIG. 4A. However, the X axis is the integrated intensity ofthe signal produced by the CCD. This graph produces results analogousthat which would be obtained using a gas chromatograph equipped with asimple non-selective detector (e.g., one that cannot provide spectra,and only uses retention time for identifying compounds).

[0098] Experimental conditions

[0099] An 80 microliter mixture of 6 compounds (acetone, methanol,cyclohexane, carbon tetrachloride, chloroform, benzene) was injectedinto the gas chromatograph 160. The mixture contained equal parts of allthe compounds except for benzene, which was present in an amount thatwas half the amount of the other compounds.

[0100] A Gow Mac 400 isothermal gas chromatograph was used. The carriergas was nitrogen, flowing at 1.4 ml/sec. The column was 4′×¼″ o.s.DC-200 on Chromasorb P WA DMCS, 80/100 mesh, at a temperature of 135degrees C.

[0101] An SPEX 1250 m. (1.25 meter) monochromator with a 150 g/mmdiffraction grating was used. The data acquisition software used wasSPEX DM3000s, which is an old DOS-based program of limited resolutionand data accumulation ability. Spectra were acquired every 2 seconds,with a slight delay (<0.5 seconds) between each spectrum. The contourplot was produced by combining 57 spectra. The total accumulation timewas 2 minutes and 8 seconds.

[0102] The 532 nm beam had an energy of approximately 1 mJ/pulse, andthe broadband beam energy generated by the OPO 142 was 4-5 mJ/pulse.

[0103] Alternate embodiments and alternate variations

[0104] It is within the scope of the present invention to substitute forthe monochromator and CCD, any spectroscopic device (monochromator orinterferometer) capable of quickly analyzing the intensity of light as afunction of wavelength, such as a Fourier Transform Interferometer,Hadamard transform spectrometer, Echelle spectrometer, and multiplestage monochromators.

[0105] It is also within the scope of the present invention tosubstitute for the gas chromatograph any separation instrument ortechnique (e.g., chromatography, electrophoresis, etc.) capable ofseparating a mixture into individual components, such as high pressureliquid chromatography, capillary electrophoresis, gel electrophoresis,ultra-centrifuge, thin-layer chromatography, liquid chromatography,column chromatography, and paper chromatography.

[0106] It is also within the scope of the present invention tosubstitute for the Nd:YAG laser any other laser or laser-like devicecapable of illuminating (optically pumping) a Raman cell to produce abeam of light at or near 683 nm, such as a Ti: sapphire laser, anExcimer laser, a Dye laser, an OPO, and a Nd: YLF laser.

[0107] In addition, it is within the scope of the present invention tosubstitute for the hydrogen-filled Raman cell producing the 683 nm beam,any device capable of generating light from 600-800 nm with sufficientenergy to illuminate (optically pump) an optical parametric oscillator,such as a Dye laser, a Ti: Sapphire laser, an Alexandrite laser, and aRuby laser.

[0108] It is within the scope of the present invention to substitute forthe OPO any device capable of generating broadband light covering acontinuous range of >3000 wavenumbers, with sufficient energy andadequate beam properties to drive a coherent Raman process, such asother optical parametric devices, an optical Parametric amplifier, awhite light generator, and a Ti: sapphire laser-based device.

[0109] It is also within the scope of the present invention tosubstitute for the narrowband 532 nm beam (that is overlapped with thebroadband light) light at any other wavelength from any other laser orlaser-like source, such as a tunable second optical parametricoscillator having a tuning range from 220 nm to 1800 nm and a bandwidthof 0.2 wavenumbers.

[0110] It is also within the scope of the present invention tosubstitute for the 10 Hz Nd:YAG laser and CCD detector any laser anddetector with different repetition rates and acquisition speeds.

[0111] It is also within the scope of the present invention tosubstitute for the 532 nm beam from the Nd:YAG laser a beam at anywavelength from any laser or similar source of light to pump the Ramancell in such a way that tilting of the Raman cell causes an enhancementof the resulting Raman-shifted beam, such as a beam of light from an OPOthat has a wavelength that can be varied from 220 nm to 1800 nm.

[0112] It is also within the scope of the present invention tosubstitute for the Nd:YAG, the hydrogen-filled Raman cell, and the OPO,any device or combination of devices capable of generating broadbandlight with sufficient energy and adequate beam properties to drive acoherent Raman process, such as dye lasers, diode lasers, diode-pumpedlasers, white light generators, and Ti:Sapphire lasers.

[0113] It is further within the scope of the present invention tosubstitute for the monochromators, any spectroscopic device(monochromator or interferometer) capable of analyzing the intensity oflight as a function of wavelength, such as a Fourier transformer, aninfrared spectrometer, and a near infrared spectrometer.

[0114] It is also within the scope of the present invention tosubstitute for the mirrors used in detectors 10 and 100, any devicecapable of reflecting or partially reflecting or redirecting a beam oflight, such as beam splitters, prisms, fiber optics, and diffractiongratings.

[0115] In addition, it is within the scope of the present invention tosubstitute for the right angle prisms, any device capable of reflectingor redirecting a beam of light, such as mirrors, fiber optics, anddiffraction gratings.

[0116] It is also within the scope of the present invention tosubstitute for the Pellin Broca prism, any device capable of spectrallypurifying a beam of light, such as diffraction gratings, other prisms,and other refractive optics.

[0117] It is also within the scope of the present invention tosubstitute for the dichroic mirrors, any device capable of reflecting abeam of light in a specific wavelength range, such as diffractiongratings, holographic optics, prisms, and fiber optics.

[0118] It is within the scope of the present invention to substitute forthe lenses used in the detectors 10 and 100, any device capable offocusing, defocusing, collimating, or changing the diameter of a beam oflight, such as prisms, curved mirrors, and Fresnel optics.

[0119] Moreover, it is within the scope of the present invention tosubstitute for the wedged window, any device capable of reflecting orextracting a portion of a laser beam, such as beam splitters, partiallyreflecting mirrors, anti-reflection coated optics, and surface of anoptic such as a prism.

[0120] It is within the scope of the present invention to substitute forthe filters, any device capable of absorbing, reflecting, or otherwiseattenuating light, such as other filters, gases, liquids, dichroicmirrors, prisms, diffraction gratings, and beam splitters.

[0121] It is within the scope of the present invention to substitute forthe iris diaphragm, any device capable of spatially selecting part of abeam of light, such as a pinhole, slit, or other aperture.

[0122] It is also within the scope of the present invention tosubstitute for the heated sample-filled Raman cells 38 and 158, anydevice capable of directing a stream of gas so that it follows a desiredpathway and permits light to enter and exit, such as glass or quartzcells, cells made of other materials such as other metals, and cellsmade with different shapes or sizes that serve the same purpose.

[0123] It is also within the scope of the present invention tosubstitute for the computer and software, the oscilloscope, thepreamplifier, and the boxcar integrator, any device capable of providingdiagnostics, data acquisition, data analysis, and electronic control ofinstrumentation, such as chart recorders, other forms of software, andphoton counters.

[0124] Further, it is within the scope of the present invention tosubstitute for the photomultiplier tube and pyroelectric detector, anydevice capable of detecting and measuring light intensity, such asphototubes, photo diodes, calorimeters, thermopiles, Si or Ge detectors,PbS or PbSe detectors, and other semiconductor detectors.

[0125] It is also within the scope of the present invention tosubstitute for the narrowband 532 nm beam, a beam of light at anywavelength that is generated by a laser-like device, such as laser or anOPO.

[0126] It is also within the scope of the present invention to useadditional beams of light, in addition to a narrowband and a broadbandbeam, in order to generate the coherent Raman signal, such as beams oflight from another laser, from an optical parametric device, from aRaman shifter, from a white light generator, and from a nonlinearoptical device.

[0127] It is also within the scope of the present invention to use thisdetection system for applications other than to determine the identityof unknown gases separated by a gas chromatograph, such as applicationsrequiring high spatial, temporal, and spectrographic resolution.Examples include combustion diagnostics, pump-probe spectroscopy,chemical vapor deposition diagnostics, laser ablation diagnostics, highresolution molecular spectroscopy, atomic spectroscopy, moleculardynamics, resonance Raman spectroscopy, electronic spectroscopy,microscopy, and other applications of gas phase spectroscopy.

[0128] It is also within the scope of the present invention to use thisdetection system or components of this system to study condensed phasesamples (liquids and solids).

[0129] It is also within the scope of the present invention tosubstitute for the dual broadband CARS method, another method to producea multiplex coherent Raman output capable of generating the entirevibrational spectrum, such as dual Stokes CARS, dual pump CARS, and dualpump-Stokes CARS.

[0130] It is also within the scope of the present invention tosubstitute for the CCD or iCCD any other device capable ofsimultaneously detecting the intensities of several portions ofspatially separated light, such as diode array detectors, multichannelplate detectors, charge injected devices, and other array detectors.

[0131] It is also within the scope of the present invention to recordspectra from regions outside of 450-530 nm. For example, the followingother regions may be recorded: 534-600 nm for viewing a region calledthe Stokes region, 530-534 nm for viewing pure rotational peaks, andadditional regions of the spectrum if the wavelength of the 532 nm beamis changed.

[0132] It is also within the scope of the present invention tosubstitute for the BBO crystals inside the OPO, any other materialcapable of generating light through a nonlinear effect, such as KDP,KTP, LiNbO₃, LiIO₃, and AgGaS₂.

[0133] It is also within the scope of the present invention tosubstitute for the OPO, any other device or method for generating abroadband beam, such as non-collinear phasematching, and pumping an OPOwith a broadband source.

[0134] It is also within the scope of the present invention tosubstitute for the half-wave plate, any other device or method foraltering the polarization of light, such as other types of wave plates,optics based on birefringent materials, and multiple-reflection devices.

[0135] It is also within the scope of the present invention tosubstitute for the diffraction gratings in the monochromators, any otherdiffraction gratings, such as diffraction gratings of different groovedensities, such as those ranging from 1 g/mm to 10000 g/mm.

[0136] It is also within the scope of the present invention tosubstitute for the monochromators used in FIGS. 1 and 3, any other typesof monochromators, such as monochromators of different focal lengths,such as those ranging from 0.06 m to 5 meters.

[0137] It is also within the scope of the present invention tosubstitute for the CCD and iCCD's used in FIGS. 1 and 3, any other typeof CCD's, including those using different pixel sizes, such as thoseranging from 1 micron to 100 microns in height and width or a differentnumber or arrangement of pixels, covering a range from 2 pixels×2 pixelsto 10000 pixels×10000 pixels. For example, a CCD with a pixel size of13.5 microns would allow the monochromater 194 to have a pixel-to-pixelresolution of 0.4 wavenumbers.

[0138] It is also within the scope of the present invention to use othermethods for combining the broadband and narrowband beams, such asnoncollinear, two dimensional or three dimensional phasematching wherean additional beam may be added or a beam may be separated into twobeams in order to allow angles to be introduced between the beams.

What is claimed is:
 1. An apparatus comprising: a narrowband coherentlight source producing a narrowband coherent beam having a bandwidth ofless than 1 wavenumber; a broadband coherent beam generator generating abroadband coherent beam having a bandwidth of more than 3000wavenumbers; and an optical device configured and positioned to directthe narrowband coherent beam and the broadband coherent beam to a samplesimultaneously to produce coherent Raman radiation scattered from thesample and comprising the complete vibrational Raman spectra of thesample with a spectral resolution of less than one wavenumber.
 2. Theapparatus defined by claim 1, wherein said broadband coherent beamgenerator generates a broadband coherent beam using the narrowbandcoherent beam as an input.
 3. The apparatus defined by claim 1, whereinsaid narrowband coherent light source comprises an optical parametricoscillator.
 4. The apparatus defined by claim 1, wherein said narrowbandcoherent light source comprises a laser.
 5. The apparatus defined byclaim 1, wherein said narrowband coherent light source produces anarrowband laser beam having a bandwidth of about 0.003 wavenumbers. 6.The apparatus defined by claim 1, wherein said broadband coherent beamgenerator generates a broadband coherent beam having wavelengths fromabout 1100 nm to about 1700 nm.
 7. The apparatus defined by claim 1,further comprising: a gas chromatograph producing the sample in the formof one or more separated gaseous species; and a sample-filled Raman cellattached to said gas chromatograph for receiving the gaseous sample,wherein said optical device directs the broadband coherent beam and thenarrowband coherent beam to said sample-filled Raman cell.
 8. Theapparatus defined by claim 1, wherein said narrowband coherent lightsource comprises an injection seeded, near transform-limited Nd:YAGlaser.
 9. The apparatus defined by claim 1, wherein said narrowbandcoherent light source comprises a Q-switched laser.
 10. The apparatusdefined by claim 1, farther comprising: a driving device configured andpositioned to produce a driving beam directed to said broadband coherentbeam generator to cause the production of the broadband coherent beamfrom said broadband coherent beam generator; and wherein said opticaldevice is positioned and configured to split said narrowband coherentbeam into first and second narrowband coherent beams, to direct thefirst narrowband coherent beam to said driving device, and to direct thesecond narrowband coherent beam to the sample.
 11. The apparatus definedby claim 10, wherein said driving device comprises a Raman cell filledwith a gas and generating a backward-propagating, phase-conjugate beamof Raman radiation comprising the driving beam in response to receivingthe first narrowband coherent beam.
 12. The apparatus defined by claim11, wherein said Raman cell is tilted with respect to an optical axisalong which the first narrowband coherent beam travels toward said Ramancell.
 13. The apparatus defined by claim 12, wherein said Raman cell istilted about 1 degree with respect to the optical axis.
 14. Theapparatus defined by claim 12, wherein said Raman cell is tilted morethan 0 degrees and less than 2.2 degrees with respect to the opticalaxis.
 15. The apparatus defined by claim 1, wherein said broadbandcoherent beam generator comprises an optical parametric oscillator. 16.The apparatus defined by claim 15, wherein said optical parametricoscillator comprises two tiltable beta barium borate crystals thatcontinuously emit broadband light in the range of 1100 to 1700 nm inresponse to receiving a driving pulse in the range of 10-130 mJ perpulse.
 17. An apparatus comprising: means for producing a narrowbandcoherent beam having a bandwidth of less than 1 wavenumber; means forgenerating a broadband coherent beam having a bandwidth of more than3000 wavenumbers; and means for directing the narrowband coherent beamand the broadband coherent beam to a sample simultaneously to producecoherent Raman radiation scattered from the sample and comprising thecomplete vibrational Raman spectra of the sample with a spectralresolution of less than one wavenumber.
 18. The apparatus defined byclaim 17, wherein said broadband coherent beam generating meansgenerates a broadband coherent beam using the narrowband coherent beamas an input.
 19. The apparatus defined by claim 17, wherein saidnarrowband coherent light source comprises an optical parametricoscillator.
 20. The apparatus defined by claim 17, wherein saidnarrowband coherent light source comprises a laser.
 21. The apparatusdefined by claim 17, wherein said narrowband coherent beam producingmeans produces a narrowband coherent beam having a bandwidth of about0.003 wavenumbers.
 22. The apparatus defined by claim 17, wherein saidbroadband coherent beam generating means generates a broadband coherentbeam having wavelengths from about 1100 nm to about 1700 nm.
 23. Theapparatus defined by claim 17, further comprising: means for performinggas chromatography that produces the sample in the form of one or moreseparated gaseous species; and means for producing Raman radiation,attached to said gas chromatography means, for receiving the gaseoussample, wherein said directing means directs the broadband coherent beamand the narrowband coherent beam to said Raman radiation producingmeans.
 24. The apparatus defined by claim 17, wherein said means forproducing a narrowband coherent beam comprises an injection seeded, neartransform-limited Nd:YAG laser.
 25. The apparatus defined by claim 17,wherein said means for producing a narrowband coherent beam comprises aQ-switched laser.
 26. The apparatus defined by claim 17, furthercomprising: driving means for driving the production of the broadbandcoherent beam from said broadband coherent beam generating means; andwherein said directing means comprises means for splitting saidnarrowband coherent beam into first and second narrowband coherentbeams, for directing the first narrowband coherent beam to said drivingmeans, and for directing the second narrowband coherent beam to thesample.
 27. The apparatus defined by claim 26, wherein said drivingmeans comprises means for generating a backward-propagating,phase-conjugate beam of Raman radiation in response to receiving thefirst narrowband coherent beam.
 28. The apparatus defined by claim 27,wherein said means for generating a backward-propagating,phase-conjugate beam of Raman radiation is tilted with respect to anoptical axis along which the first narrowband coherent beam travelstoward said means for generating a backward-propagating, phase-conjugatebeam of Raman radiation.
 29. The apparatus defined by claim 28, whereinsaid means for generating a backward-propagating, phase-conjugate beamof Raman radiation is tilted about 1 degree with respect to the opticalaxis.
 30. The apparatus defined by claim 28, wherein said means forgenerating a backward-propagating, phase-conjugate beam of Ramanradiation is tilted more than 0 degrees and less than 2.2 degrees withrespect to the optical axis.
 31. The apparatus defined by claim 17,wherein said broadband laser beam generating means comprises an opticalparametric oscillator.
 32. The apparatus defined by claim 31, whereinsaid optical parametric oscillator comprises two tiltable beta bariumborate crystals that continuously emit broadband light in the range of1100 to 1700 nm in response to receiving a driving pulse in the range of10-130 mJ per pulse.
 33. A method of generating the complete vibrationalRaman spectra of a sample comprising the steps of: producing anarrowband coherent beam having a bandwidth of less than 1 wavenumber;generating a broadband coherent beam having a bandwidth of more than3000 wavenumbers; and directing the narrowband coherent beam and thebroadband coherent beam to a sample simultaneously to produce coherentRaman radiation scattered from the sample and comprising the completevibrational Raman spectra of the sample with a spectral resolution ofless than one wavenumber.
 34. The method defined by claim 33, furthercomprising the step of using the narrowband coherent beam to generatethe broadband coherent beam.
 35. The method defined by claim 33, whereinsaid producing step comprises the step of producing the narrowbandcoherent beam with a laser to produce a narrowband laser beam.
 36. Themethod defined by claim 33, wherein said producing step comprises thestep of producing the narrowband coherent beam with an opticalparametric oscillator laser to produce the narrowband coherent beam. 37.The method defined by claim 33, wherein said producing step produces anarrowband coherent beam having a bandwidth of about 0.003 wavenumbers.38. The method defined by claim 33, wherein said generating stepgenerates a broadband coherent beam having wavelengths from about 1100nm to about 1700 nm.
 39. The method defined by claim 33, furthercomprising the steps of: performing gas chromatography to produce thesample in the form of one or more separated gaseous species; anddirecting the broadband coherent beam and the narrowband coherent laserbeam to the gaseous sample.
 40. The method defined by claim 33, whereinsaid producing step is performed with an injection seeded, neartransform-limited Nd:YAG laser.
 41. The method defined by claim 33,wherein said producing step is performed with a Q-switched laser. 42.The method defined by claim 33, further comprising the steps of: causingthe production of the broadband laser beam from a broadband laser beamsource with a driving device; splitting the narrowband coherent beaminto first and second narrowband coherent beams; directing the firstnarrowband coherent beam to the driving device; and directing the secondnarrowband coherent beam to the sample.
 43. The method defined by claim42, wherein said causing step comprises the step of generating abackward-propagating, phase-conjugate beam of Raman radiation with thedriving device in response to the driving device receiving the firstnarrowband coherent beam.
 44. The method defined by claim 43, whereinsaid causing step further comprises the step of tilting the drivingdevice with respect to an optical axis along which the first narrowbandcoherent beam travels toward the driving device.
 45. The method definedby claim 44, wherein said tilting step comprises the step of tilting thedriving device about 1 degree with respect to the optical axis.
 46. Themethod defined by claim 44, wherein said tilting step comprises the stepof tilting the driving device more than 0 degrees and less than 2.2degrees with respect to the optical axis.
 47. The method defined byclaim 33, wherein said generating step comprises the step of generatingthe broadband coherent beam with an optical parametric oscillator.
 48. ARaman cell for generating a driving pulse for a broadband coherent beamgenerator comprising: a closed cell filled with gas that produces abackward-propagating, phase-conjugate, coherent Raman radiation beam ofsubstantially circular cross-section and substantially uniform intensityin response to being irradiated with a coherent beam, wherein saidclosed cell has side walls, wherein said closed cell has a windowthrough which the coherent beam can enter to irradiate the gas insidethe closed cell, and wherein said closed cell is tilted with respect tothe optical axis along which the coherent beam travels to the window sothat the entire coherent beam enters the window and is focused on one ofthe side walls of said closed cell.
 49. The Raman cell defined by claim48, wherein said closed cell is tilted about 1 degree with respect tothe optical axis.
 50. The Raman cell defined by claim 48, wherein saidclosed cell is tilted more than 0 degrees and less than 2.2 degrees withrespect to the optical axis.
 51. The Raman cell defined by claim 48,wherein said closed cell produces a 683 nm beam of about 70 mJ inresponse to being irradiated by a 532 nm laser beam of about 200 mJ. 52.The Raman cell defined by claim 48, wherein said closed cell producesthe backward-propagating, phase-conjugate, coherent Raman radiation beamof substantially circular cross-section and substantially uniformintensity in response to being irradiated with a coherent beam having awavelength in the range of 531.7 nm to 532.3 nm.
 53. The Raman celldefined by claim 48, wherein said closed cell produces thebackward-propagating, phase-conjugate, coherent Raman radiation beam ofsubstantially circular cross-section and substantially uniform intensityin response to being irradiated with a coherent beam having a wavelengthof any value.
 53. The Raman cell defined by claim 48, wherein saidclosed cell produces the backward-propagating, phase-conjugate, coherentRaman radiation beam of substantially circular cross-section andsubstantially uniform intensity in response to being irradiated with acoherent beam having a wavelength from 200 nm to 20,000 nm.